Conventional swing control systems for working machines include one arranged on a hydraulic excavator. This conventional swing control system is provided with a swing mechanism for driving an upperstructure by an output torque of an electric motor rotatable in opposite two directions, a swing operating device (control lever) selectively operable in opposite two directions from a neutral position and capable of converting a operation direction and operation stroke to a swing command signal, and a control means for controlling the electric motor based on the swing command signal from the swing operating device.
The control means is provided with a rotation speed detection means for detecting a rotation speed of the electric motor, a target rotation speed calculation means for calculating, based on the swing command signal from the swing operating device, a target rotation speed for the electric motor, a speed deviation calculation means for calculating a speed deviation of an actual rotation speed, which has been detected by the rotation speed detection means, relative to the target rotation speed, and a first target torque calculation means for calculating a target torque for the electric motor in a direction that the speed deviation calculated by the speed deviation calculation means will be eliminated. By these means, feedback control is performed on the rotation speed of the electric motor. As a result, in each of the opposite two directions of operation of the swing operating device from the neutral position, the swing speed of the upperstructure is controlled more as the operation stroke from the neutral position increases, and the swing speed of the upperstructure is controlled less as the operation stroke decreases.
Kinds of work performed by a hydraulic excavator includes pressing work. This pressing work is work that compacts and shapes an inner side wall of a ditch by pressing an outer side wall of a bucket of front working equipment against the inner side wall of the ditch. During this pressing work, the upperstructure does not turn, in other words, the actual rotation speed does not change from 0. Only with feedback control that controls the output torque of the electric motor to the target torque calculated by the first target torque calculation means, the output torque of the electric motor is, therefore, in a state that it is maintained at substantially the maximum irrespective of the operation stroke of the swing operating device. In this state, it is impossible to adjust the output torque of the electric motor in a direction that the bucket is pressed against the inner side wall of the ditch.
The control means is, therefore, provided with a second target torque calculation means for calculating, based on the swing command signal from the swing operating device, a target torque for the electric motor in the same direction as the target rotation speed, a detection means for detecting a state that the speed deviation has reached a predetermined value or greater, in other words, a state that pressing work has been started and turn has become stagnant, a switch means for automatically switching the control of the electric motor from a state, in which the control of the electric motor is performed based on the target torque calculated by the first target torque calculation means, to a state in which the control of the electric motor is performed based on the target torque calculated by the second target torque calculation means. During the pressing work, the output torque of the electric motor is controlled by these means based on the target torque calculated by the second target torque calculation means. As a result, in each of the opposite two directions of operation of the swing operating device from the neutral position, the output torque of the electric motor is controlled more as the operation stroke from the neutral position increases, and the output torque of the electric motor is controlled less as the operation stroke decreases. In other words, it has become possible to adjust the output torque of the electric motor in the direction that the bucket is pressed against the inner side wall of the ditch (see Patent Document 1).